The scene's told from Cinderella's point of view. She sees this mess and chaos of ripping where her stepsisters are acting like cruel feral beasts. The exaggeration here is to help the viewers empathise with how much this is emotionally affecting Cinders.

I think it was due to the fact that the sisters were very angry and if they destroyed her dress she won't be going to the ball. Also the main portion of the dress was old. It was made from a dress that Cinderella's mother had/wore. It was only made up and embellished to make it into a more modern dress for Cinderella.